1. Field of the Invention
The present invention relates to an activation decision apparatus for measuring an impact force or an acceleration applied to a vehicle and determining whether or not passive restraints, such as air bag units, mounted on the vehicle are to be activated, based on the results of measurement.
2. Description of the Prior Art
As a recent trend, passive restraints, such as air bag units and seat belts with pre-loaders, are generally mounted on a vehicle for the purpose of protecting a driver and passengers from damages in a crash. When a large impact is applied to the vehicle in a collision and an activation decision apparatus mounted on the vehicle determines that passive restraints are to be activated, an ignition device, such as a squib, ignites a gas-generating agent and thereby activates the passive restraints. The ignited gas-generating agent evolves a gas, and the pressure of the gas drives the passive restraints.
A conventional activation decision apparatus measures an impact applied to the vehicle, for example, with an acceleration sensor as an acceleration, compares the observed acceleration with a predetermined threshold value, and determines whether or not the passive restraints are to be activated, based on the results of comparison. For example, when a large impact is applied to the vehicle in a crash, the observed acceleration exceeds the predetermined threshold value, so that the passive restraints are activated. When a small impact is applied, on the other hand, the observed acceleration does not exceed the predetermined threshold value, so that the passive restraints are not activated. In the structure of determining activation or inactivation of passive restraints based on the magnitude of the observed acceleration, however, even in case of an underside hit when a greater impact is applied to the vehicle than in case of a collision of the vehicle, the observed acceleration exceeds the predetermined threshold value and the apparatus thereby determines that the passive restraints are to be activated. The proposed techniques accordingly do not use the observed acceleration itself for the decision, but integrate the acceleration and determine activation or inactivation of passive restraints based on the magnitude of the integral value (for example, `Activation Apparatus for Passive Restraints on Vehicle` disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 4-325348).
The term `underside hit` in the description hereof implies that the vehicle runs on to a curb or an obstacle such as stone on the road or is caught in a depression or a hole in the road and a lower portion of the vehicle (including the lower face of the engine, the suspension, the lower face of the oil pan, and the wheels) thereby comes into contact with the obstacle.
FIGS. 11(a) through 11(d) show the process of determining activation or inactivation of passive restraints by the conventional activation decision apparatus in case of an underside hit and in case of a collision of the vehicle. FIG. 11(a) shows the waveform of the observed acceleration signal in case of an underside hit; FIG. 11(b) the integral waveform of FIG. 11(a); FIG. 11(c) the waveform of the observed acceleration signal in case of a collision of the vehicle; and FIG. 11 (d) the integral waveform of FIG. 11(c). The data of FIGS. 11(a) and 11(c) are plotted, with acceleration as ordinate and time `t` as abscissa, wherein the accelerations applied backward with respect to the vehicle are defined as the positive side and those applied forward are defined as the negative side. The data of FIGS. 11(b) and 11(d) are plotted, with integral value as ordinate and time `t` as abscissa, wherein Th on the ordinate represents a threshold value that is used as a criterion for determining activation or inactivation of passive restraints.
A collision of the vehicle is generally a non-elastic collision that accompanies deformation of the body. When the vehicle collides in a frontal crash, the accelerations applied to the vehicle occur only on the positive side (that is, backward with respect to the vehicle) as shown in FIG. 11(c). The accelerations generated before final determination of activation or inactivation of passive restraints are due to a deformation of the body on the frontal side of the vehicle and are accordingly not significantly large.
Integration of the acceleration gives a curve of the integral value as shown in FIG. 11(d) and activates the passive restraints at a time point t11, when the integral value exceeds the threshold value Th.
In case of an underside hit, on the other hand, although the energy of impact is relatively small, since the impact continues for a very short time period, accelerations applied to the vehicle are significantly large. Such an underside hit is an elastic collision, and the observed accelerations remarkably fluctuate both on the positive side (that is, backward with respect to the vehicle) and on the negative side (that is, forward with respect to the vehicle) as shown in FIG. 11(a). Since a greater impact is applied backward with respect to the vehicle than forward when the vehicle runs forward, the amplitude on the positive side becomes larger than the amplitude on the negative side (that is, the acceleration includes a positive DC component).
An absolute value of the integral of the acceleration on the positive side is accordingly larger than that on the negative side. The overall integral value (that is, the sum of the integral value on the positive side and the integral value on the negative side) thus gradually increases and eventually exceeds the threshold value Th at a time point t10 as shown in FIG. 11(b).
The known structure that integrates the observed acceleration and determines activation or inactivation of passive restraints based on the magnitude of the integral value can not appropriately discriminate the case of activation as shown in FIG. 11(d) from the case of inactivation as shown in FIG. 11(b).